A Case of Neutropenic Enterocolitis During Endocrine Therapy With Abemaciclib for Metastatic Breast Cancer

BACKGROUND/AIM: Abemaciclib (ABE), a selective cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor, is a crucial first-line treatment for hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer. While known side effects include diarrhea, neutropenia, fatigue, nausea and anemia, severe complications are generally rare. Our report describes the occurrence of neutropenic enterocolitis (NE) in a patient undergoing ABE therapy for bilateral breast cancer complicated by bone marrow carcinomatosis. CASE REPORT: A premenopausal woman in her 50s was diagnosed with bilateral breast cancer with bone marrow carcinomatosis. Apart from anemia, her general condition was stable at treatment initiation. Two and a half months after starting treatment with ABE, a luteinizing hormone-releasing hormone agonist, and an aromatase inhibitor, she presented to the emergency department with right lower abdominal pain and fever. The patient was in shock, and computed tomography revealed circumferential wall thickening of the colon. Her neutrophil count was critically low at 120/μl, leading to a diagnosis of NE. She required three days of intensive care with continuous noradrenaline infusion. CONCLUSION: This case highlights a rare but severe complication of ABE treatment. While CDK4/6 inhibitors are widely used as first-line agents in advanced breast cancer due to their high efficacy and generally favorable side-effect profile, clinicians should remain vigilant for NE, which requires prompt intervention.